The team also re-assigned left-hander Patrick McCoy and infielders Will Rhymes and Matt Skole to Minor League camp. The Nationals have 42 players in their big league camp after the moves made prior to Thursday's game against the Astros.

Rendon, Skole and Karns stood out while in big league camp. Rendon hit .375 and led the team with four home runs and 11 RBIs. The sixth overall pick in the 2011 First-Year Player Draft is expected to become Harrisburg's everyday third baseman, but will also see time at shortstop and second base.

He must show that he can stay healthy this season after playing only 43 games last year because of an ankle injury.

"I thought I did really well [in Spring Training]," Rendon said. "I wanted to come here and make an impact on the team and make them remember me. I have to stay consistent. I have to go in there, work hard, get my work in, just get ready for the season."

Skole hit .250, but he led the team with seven walks and had a .406 on-base percentage. He acknowledged that he was sad to leave the big club.

"I knew this was going to come. I knew that coming in," said Skole, who was Washington's Minor Player of the Year last season with Class A Hagerstown and Class A Advanced Potomac. "I learned a lot over here not only about myself, but about the organization, about who they are as people. I just had a blast. I can't be more fortunate to have this opportunity. I thank Davey and the Nationals for giving me this chance and learn what this organization has to offer."

After having a great first game against the Mets on Feb. 25, striking out three and allowing one hit in two innings, Karns suffered tightness in his upper right leg on March 2. As the injury got worse, Karns was inconsistent on the mound and allowed seven runs -- one earned -- against the Cardinals last Friday. The Nationals shut him down for the past week, but he is expected to have a bullpen session Friday and then play in Minor League games.

"I feel really good about it. I have a bullpen session tomorrow. It's the first step of our plan of attack," Karns said. "Right now, I'm focused on getting back to work. I missed a lot of valuable time, but I still have some experience that I'm going to build on. I'll go to the Minor Leagues and go right back at it, right back to work."

Johnson acknowledged that he has concerns about Perez's participation in the World Baseball Classic with the Dominican Republic, which plays the United States on Thursday at 7 p.m. ET. Perez is not getting any playing time, and Johnson wants him to get at-bats once he returns to Minor League camp.